As explained by ConsumerLab.com's president, Tod Cooperman, M.D. in a New York Times article
, the "Best By" or "Use By" date on a supplement label indicates how long a supplement is expected to last before its potency falls below 100% of the listed amount. Although the FDA does not require supplement labels to provide an expiration date, companies which include these dates are required to have stability data to support their claims. The expiration date refers to the shelf life of the unopened container stored under the conditions specified on the label.
A "Date of Manufacture" simply indicates when the supplement was made, not how long the ingredients remain stable and potent. Typically, supplements are stable and potent if properly maintained for two to three years, but this depends on the product. Exposure to heat and/or moisture can significantly shorten shelf life.
Be sure to take extra care when looking at the date listed on probiotics supplements. Some may list the number of cells in a product as the amount "at the time of manufacture" — a practice which is misleading and violates FDA regulations. You can learn more about this in the "ConsumerTips" section of ConsumerLab.com's Probiotic Supplements Review
, as well as in this interview with Dr. Tod Cooperman
on Reuter's Health.